Method and apparatus for a fuel-rich catalytic reactor

ABSTRACT

The present invention is a method, and an apparatus for practicing the method, that creates a product stream and a heat of reaction from a fuel-rich fuel/air mixture and then contacts the product stream with a sufficient quantity of additional air to completely combust all of the fuel, to which air a portion of the heat of reaction has been transferred.

CROSS-REFERENCE TO OTHER APPLICATION

[0001] This application is a continuation of co-pending application Ser.No. 09/909,517 that is a Divisional Application of application Ser. No.09/527,708, which has issued as U.S. Pat. No. 6,358,040. All of whichare incorporated herein in their entirety by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a catalytic reactor that may beemployed in a variety of uses, such as for gas turbine enginecombustion, or for other combustion systems. More particularly, thepresent invention is directed to a method that creates a product streamand a heat of reaction from a fuel-rich fuel/air mixture and thencontacts the product stream with a sufficient quantity of additional airto completely combust all of the fuel and to which a portion of the heatof reaction has been transferred.

[0004] 2. Brief Description of the Related Art

[0005] At high temperature, particularly above approximately 2800degrees F., the oxygen and nitrogen present in air combine to form thepollutants NO and NO₂, collectively known as NOx. As flame temperaturesof most fuels reacting with air can easily exceed this value, a goal ofmodern combustion systems is to operate at reduced temperatures, so thatsuch thermal formation of NOx is limited.

[0006] Reduced-temperature combustion is typically accomplished bypremixing the fuel with sufficient excess air that the flame temperatureis reduced to a value at which thermal NOx production is minimal(typically a temperature below approximately 2800 degrees F.). At theselower flame temperatures, however, the rate of combustion may beinsufficient to prevent localized or global blow-off or extinction,particularly under conditions of turbulent flow. Flame anchoring andflame stability thus become problematic at the lower flame temperaturesrequired for truly low-NOx lean-premixed combustion. Thus, achievableNOx reduction is limited.

[0007] A commonly-employed solution to the problems of flame anchoringand flame stability is to react a portion of the fuel at a highertemperature, and to then use the resulting high-temperature gases toinitiate, sustain, and stabilize (“pilot”) the lower-temperaturecombustion of the main fuel/air mixture. The higher-temperature “pilot”combustion zone can take various forms, and can be fuel-lean orfuel-rich (the fuel-rich pilot case is typically known asRich-burn/Quench/Lean-burn or RQL combustion). In either case,undesirable NOx generation results from operation of the pilot. For thefuel-lean case, NOx generation occurs within the pilot flame as a resultof the high flame temperature required for stable pilot operation. Forthe fuel-rich case, the oxygen-deficiency of the pilot's fuel-richenvironment is not favorable to NOx formation within the pilot; however,NOx generation occurs when the high-temperature highly-reactive mixtureexiting the pilot contacts (and reacts with) the additional air requiredto complete combustion of the fuel.

[0008] An alternative method of stabilizing combustion, withouthigh-temperature piloting, is to employ a catalyst. Because a catalystallows stable low-temperature reaction of fuel and air, all or a portionof the fuel can be reacted at a moderate temperature without NOxgeneration. The pre-reaction of a portion of the fuel stabilizes themain combustion process by providing preheat, reactive species from fuelpartial oxidation or fragmentation, or both. This type of system isreferred to herein as “catalytically stabilized combustion,” or simply“catalytic combustion.”

[0009] While the effectiveness (stability and low pollutant emissions)of catalytic combustion is well documented and well known in the art,commercial development of catalytic combustion requires resolution ofunique new design issues introduced by the catalytic reactor. Dominantamong these issues is the need to operate the catalyst at a safetemperature.

[0010] For example, in gas turbine engine applications having highturbine inlet temperatures, the adiabatic flame temperature of the finalfuel/air mixture generally exceeds catalyst and/or substrate materialtemperature limits. Accordingly, there is a need to control and limitcatalyst operating temperature to a value below the final adiabaticflame temperature. Thus, only a portion of the total fuel can be reactedin the catalyst bed. For high thermal efficiency in a heat engine, suchas a gas turbine engine, such control and limitation of catalystoperating temperature must occur without net heat extraction from theengine's working fluid.

[0011] While it is possible to stage the fuel injection, so that only aportion of the fuel passes through the catalyst bed (with the remainderbeing injected downstream), issues arise of fuel injection and mixing inthe hot-gas path downstream of the catalyst. Thus, it is generallyconsidered preferable to pass all of the fuel through the catalyst bed.Such use of a single fuel stage, however, requires some means ofensuring only partial combustion of the fuel passing through thecatalyst bed. Depending upon catalyst operating conditions, additionalmeans of limiting the catalyst operating temperature may also berequired, such as catalyst cooling by combustion air or fuel or both.

[0012] One approach to limiting combustion of the fuel/air mixture inthe catalyst bed is presented in U.S. Pat. No. 5,235,804 (to Colket etal.). The '804 patent teaches partially reacting the fuel in a fuel-richfuel/air mixture in the catalyst bed, the reaction being limited by aninsufficiency of oxygen to completely convert all the fuel to CO₂ andH₂O. In the '804 patent, the catalytic reaction is intended to provideboth flame stability enhancement to the primary (gas-phase) combustionzone and a means for thermal management. Thermal management means that aportion of the heat of catalytic reaction is extracted from thecombustion system, permitting a reduction of the flame temperature inthe primary combustion zone, and consequently a reduction in NOxformation.

[0013] Because a primary goal of the '804 patent's system is to reduceflame temperature in the primary combustion zone by extracting a portionof the heat of reaction before the primary combustion zone, a keyfeature of this system is the use of a bypass air stream to provide theoxygen required for combustion completion in the primary combustionzone. This bypass air stream does not obtain heat from the heat ofreaction within the catalytic oxidation stage. Thus, a third stream isrequired for catalyst cooling.

[0014] The need for separate cooling and combustion air streamsintroduces several disadvantages. For example, in a gas turbine engineoperating with a turbine inlet temperature at or only slightly lowerthan the maximum combustion temperature for low NOx emissions, low NOxoperation requires that virtually all the compressor air enter theprimary combustion zone to limit combustion temperature, with little orno dilution air added to the combustor effluent before the turbine.Thus, little or no compressor air would be available for catalystcooling in the '804 system. If sufficient cooling air were madeavailable to the catalyst, the turbine inlet temperature would belimited to a value significantly lower than the maximum low-NOxcombustion temperature by addition of this cooling air downstream ofcombustion. Alternatively, catalyst cooling air could exit the systemwithout passing through the turbine, resulting in a system loss of theheat of reaction extracted by the catalyst cooling stream, and a loss ofengine efficiency.

[0015] In either case, the catalyst cooling air, which will be in closecontact with the catalyzed fuel/air mixture during cooling, must bedirected around the primary combustion zone while the catalyzed fuel/airmixture is directed into the primary combustion zone. The bypass airmust be directed around the catalyst bed and then into the primarycombustion zone. While this routing is not prohibitive, it doesintroduce hardware complexity, space requirements, and design challengesto the overall combustion system

[0016] It has now been found that a system employing a fuel-richcatalytic reaction, with transfer of a portion of the heat of reactionto the ultimate combustion air stream (not to a separate coolingstream), can provide low-NOx combustion with enhanced combustionstability along with well-moderated catalyst operating temperatures andcomplete use of the fuel heating value. By utilizing the ultimatecombustion air for catalyst cooling, sufficient cooling air is ensuredregardless of the final burner outlet temperature (or turbine inlettemperature in a gas turbine engine).

SUMMARY OF THE INVENTION

[0017] The present invention is a method, and an apparatus, for reactinga mixture of fuel and oxidizer (a “fuel/oxidizer mixture”). Theinvention was developed using a hydrocarbon fuel and air, which containsthe oxidizer oxygen, therefore for clarity of presentation of theinvention the more conventional fuel/air terminology (“fuel/airmixture”) will be used, but the invention should not be considered solimited.

[0018] The term “equivalence ratio” is used to denote the proportions offuel and air in a fuel/air mixture. The equivalence ratio is the ratioof the actual fuel/air ratio to the stoichiometric fuel/air ratio, wherethe stoichiometric coefficients are calculated for the reaction givingfull oxidation products CO₂ and H₂O. An equivalence ratio greater than1.0 defines a fuel-rich fuel/air mixture, and an equivalence ratio lessthan 1.0 defines a fuel-lean fuel/air mixture.

[0019] In the basic method of the present invention, a fuel-richfuel/air mixture is contacted with a catalyst to oxidize a portion ofthe fuel contained therein. The catalytic reaction provides both a heatof reaction and a product stream. A portion of the heat of reaction isconducted to a cooling air stream and the product stream then contactedwith the heated cooling air. The term product stream as used hereinmeans the effluent from the fuel-rich fuel oxidation process comprisingthe remaining fuel values after reaction of the entering fuel/airmixture, where the remaining fuel values can include residual fuel(entering fuel unchanged) and/or fuel partial oxidation products(entering fuel partially oxidized but less than fully combusted).

[0020] As recognized in the art, hydrocarbons and most other fuels havea limited range of fuel/air ratios within which a flame can propagate.The rich flammability limit is the highest equivalence ratio for flamecombustion and the lean flammability limit is the lowest. As is known,these limits typically widen with increase in mixture temperature. Thecatalytic reaction of the present invention, unlike flame combustion, isnot limited to equivalence ratios within the flammability limits.

[0021] Thus fuel-rich equivalence ratios of ten or higher may beutilized in the present invention. An equivalence ratio of 10, however,seems to be a practical maximum beyond which little heat output isobtained from the catalytic reactor. In the method of the presentinvention the fuel/air mixture in contact with the catalyst is fuel richand thus the amount of oxygen available, determined by the equivalenceratio of the fuel-rich fuel/air mixture, limits the extent of reactionand heat release possible. An equivalence ratio of no more than about 5is usually preferred. At very high equivalence ratios, greater than 10for example, carbon may accumulate on some catalyst types, in which caseperiodic regeneration may be required to burn off accumulated carbon.

[0022] Because the product stream composition may vary depending uponcatalyst selectivity (H₂ and CO versus H₂O and CO₂), the amount of fuelconverted for a given amount of oxygen consumed depends on catalystselectivity. Thus, for the purposes of this invention conversion withinthe catalytic reactor, unless otherwise stated, refers to the fractionof oxygen within the fuel-rich fuel/air mixture consumed prior to themixture's exit from the catalytic reactor.

[0023] It is a requirement of the method that a portion of the heat ofreaction be conducted into a cooling fluid stream thereby causing atemperature rise in the cooling fluid. Common methods of accomplishingthis heat transfer are by a heat exchanger within or downstream of thecatalyst zone or by backside cooling of the catalyst. Backside coolingis a technique whereby the catalyst is on one side of a substrate andthe cooling fluid stream is brought into contact with the other side ofthe substrate.

[0024] Backside cooling allows the catalyst to operate at a temperaturelower than the adiabatic flame temperature of the fuel-rich fuel/airmixture, even when the catalytic reactor is operated in a mass transferlimited regime, and thus is useful for controlling the temperature ofcatalyst and substrate materials having maximum operating temperatureslower than the reaction mixture's adiabatic flame temperature. Backsidecooling is not needed for oxidation of fuel/air mixtures havingadiabatic flame temperatures less than the safe operating temperature ofthe catalyst employed. A catalytic reactor is said to operate in a masstransfer limited regime when the catalytic reaction rate is sufficientlyfast that the net rate of conversion of reactants is limited by masstransfer of reactants from the bulk fluid stream to the catalystsurface. For a fluid stream with an effective Lewis number near unity(ratio of thermal diffusivity to mass diffusivity), a catalyst surfaceoperating in a mass transfer limited regime will reach temperatures nearthe adiabatic flame temperature of the reaction mixture unless coolingis provided.

[0025] It is also a requirement of the method that the cooling fluidstream be of sufficient flow rate that if it were mixed with the productstream the resulting mixture would be a fuel-lean fuel/air mixture. Ifdesired additional air may be added with the cooling stream to form thefuel-lean fuel/air mixture. In the method air performs two functions. Asa first fluid it provides an oxidizer to support the catalyticcombustion of the fuel, and as a cooling fluid it acts as a heat sink.The first fluid and the cooling fluid can be from different sources orfrom a common source, such as a third fluid from which is separated thefirst fluid and the cooling fluid.

[0026] After catalytic reaction of the fuel/air mixture stream, theproduct stream and the cooling fluid stream are brought into contact.After contact, several alternate steps are possible. The firstalternative after contact is to mix the product stream and the coolingstream to create a fuel-lean fuel/air mixture. Mixing is defined hereinto mean that the two components, product steam and cooling stream, aremade into a single collection, to the mixedness desired, prior toinflammation. The inflammation limitation does not mean thatinflammation is entirely prohibited during mixing, but instead meansthat chemical reactions or isolate inflammation may be present, but notto a degree that would cause an all consuming inflammation withsubstantial reaction of the product stream's remaining fuel values.

[0027] While isolate inflammation is allowable, for minimum NOxformation it is preferred that such isolate inflammation be negligibleor absent. Pre-inflammation reactions, occurring in the gas-phase but atslow rates and low temperatures compared to actual inflammation, do notimpact NOx and may be present during mixing as a result of the catalysteffluent's high reactivity.

[0028] It is a significant discovery that high-temperature, non-premixedburning can be prevented, without net heat extraction from thecombustion system, during the mixing of a partially-combusted mixturewith air for final combustion. Non-catalytic attempts at similarprocesses (particularly RQL, Rich-burn/Quench/Lean-burn combustion) haverequired high temperatures to support gas-phase reaction during thefuel-rich partial combustion process, and have consequently been unableto prevent high-temperature burning during the subsequent mixingprocess. In the present invention, the catalytic reactor's productstream may exit at a significantly lower temperature since oxidationoccurs catalytically instead of in the gas-phase, with the result thatmixing may occur without burning. However, stability is still impartedto any downstream combustion process, via preheat, the generation ofreactive species from fuel partial oxidation or fragmentation, or both.

[0029] To ensure that inflammation does not occur during mixing of theproduct stream with the heated cooling stream, both flameholding andpremature auto-ignition should be prevented. Flameholding can beprevented by standard methods known in the art, particularly by ensuringadequate flow velocity and a streamlined flow path (free fromrecirculation zones) in the region where the product stream and theheated cooling stream mix. Premature auto-ignition is prevented bycompleting the mixing process in a time that is less than the time forauto-ignition. Thus, both mixing time and auto-ignition delay time mustbe considered.

[0030] Mixing time can be determined by methods known in the art, suchas direct measurement, or analytical calculation or computational fluiddynamics (CFD) utilizing models of turbulent flow if appropriate.Auto-ignition delay time is more difficult to determine, but can beestimated based on data and models widely available in the combustionliterature. One difficulty is that auto-ignition delay time is generallydefined for a fixed equivalence ratio, while mixing processes bydefinition encompass a wide range of equivalence ratios. Fortunately,however, the dependence of auto-ignition delay time on equivalence ratiois weak, with fuel type (mixture composition), temperature, and pressurebeing the more determinative factors. Thus, it will be found that bydesign and in accordance with this disclosure, it is straightforward toachieve such mixing without auto-ignition. The example of the inventiondisclosed herein demonstrates one application of the method anddescribes one apparatus for realizing the method.

[0031] Depending upon the specific design requirements of the catalyticreactor, to facilitate mixing of the product stream and cooling streamto a fuel-lean fuel/air mixture, interspersion of the two streams may beemployed. Interspersion introduces immediate small-scale mixing of thecooling fluid stream with the product stream, and can allow for rapidmixing without inflammation by assuring that mixing occurs in a shortertime than the auto-ignition delay time. The product stream may besubdivided and interspersed into the cooling stream; the cooling streammay be subdivided and interspersed into the product stream; or both maybe subdivided and interspersed. Advantageously, the product and coolingstreams exit at different velocities to create a highly sheared layerpromoting rapid mixing and a high strain rate inhibiting inflammation.

[0032] In this method of operation, preferably at least about 50 percentof the heat of reaction is conducted into the cooling fluid stream. Fora backside-cooled catalyst having sufficient cooling fluid flow, thispercentage of heat transfer is readily achievable. This heat transfermoderates the temperature of the product stream prior to contact of theproduct stream with the cooling fluid stream, advantageously increasingthe auto-ignition delay time before inflammation. In this mode ofoperation, the exiting product stream temperature must be low enough toallow a finite time to achieve mixing prior to inflammation.

[0033] As a further step in the method, the fuel-lean fuel/air mixture,if within combustible limits, can be combusted in the gas phase. Whetherthe fuel-lean fuel/air mixture is within combustible limits will dependon the resulting temperature, composition (absence or presence ofpartial oxidation products such as H₂), and equivalence ratio of thefuel-lean fuel/air mixture. The mixture will be combustible if theequivalence ratio is above the equivalence ratio corresponding to themixture's lean flammability limit at the mixture temperature. Methods todetermine flammability limits are known. Depending upon the degree ofoxidation and the amount of the heat of reaction, gas-phase combustionmay be achieved through auto-ignition or other ignition source.

[0034] A second alternative after contact is to allow inflammation uponcontact, without mixing. Unlike the first alternative, this secondmethod of operation does not require an ignition delay prior to completeinflammation. In this method of operation the combustion temperature atthe stoichiometric interface between the product stream and the heatedcooling fluid stream is advantageously reduced sufficiently to limit NOxproduction. It has been found that by transferring sufficient heat fromthe fuel-rich product stream to the cooling air stream before contact,the adiabatic flame temperature at the stoichiometric interface betweenthe product stream and the cooling air stream can be reduced to a valuewell below the adiabatic flame temperature that would exist at thestoichiometric interface in the absence of heat transfer between thestreams. Thus, NOx formation can be limited even if stoichiomericburning occurs during mixing.

[0035] The mechanism for this reduction in stoichiometric interfaceflame temperature is conduction of heat, but not mass (fuel oroxidizer), between the product stream and the cooling fluid stream.Thus, while the proportions of the product stream and the cooling airstream required to create a stoichiometric mixture are not affected byheat transfer between the streams, the initial temperature (beforereaction) of a stoichiometric mixture of the two streams can besignificantly lowered; accordingly, the resulting adiabatic flametemperature can also be significantly lowered.

[0036] As an example, for the purpose of illustration, let a fuel-richequivalence ratio only infinitesimally higher than 1.0 be completelyreacted in contact with the catalyst, and let sufficient cooling air beprovided such that the overall fuel-lean equivalence ratio of thecombined product stream and cooling air stream is 0.5. Further, assumethat thermal equilibrium between the streams is obtained before thestreams contact each other. In this example, the adiabatic flametemperature at the stoichiometric interface between the contactingstreams will be nearly equal to the adiabatic flame temperature at 0.5equivalence ratio, and near-zero NOx emissions will result fromstoichiometric burning during mixing. If no heat had been transferredbetween the streams, however, the adiabatic flame temperature at thestoichiometric interface would not be reduced, and would remain equal tothe adiabatic flame temperature for a stoichiometric mixture of theinlet fuel and air.

[0037] The adiabatic flame temperature at the stoichiometric interfacedepends directly on the temperature and composition of the productstream and the heated cooling fluid stream, and thus depends indirectlyon the heat of reaction in the catalytic reactor, on the portion of theheat of reaction transferred to the cooling fluid stream, and on thethermal capacity (product of mass flow rate and heat capacity) of eachof the two streams. Given these operating conditions, calculation of theadiabatic flame temperature at the stoichiometric interface isstraightforward. In particular, one need only calculate, by analyticalor numerical methods, the composition and temperature at chemical andthermal equilibrium of a stoichiometric mixture of the product streamand the heated cooling fluid stream. The composition and temperature ofthe product stream and the heated cooling stream, before mixing andequilibration, can be found either experimentally or by heat and masstransfer calculations (including turbulence modeling if appropriate)standard in chemical engineering practice. Note that for a givenfuel-rich equivalence ratio, the heat of reaction in the catalyticreactor will depend on the selectivity of the catalyst to full oxidationproducts (CO₂ and H₂O) or partial oxidation products (CO and H₂),partial oxidation products providing a lower heat of reaction than fullpartial oxidation products.

[0038] It is a significant discovery that the method of the presentinvention by conduction of a portion of the heat of reaction to thecooling fluid lowers the adiabatic flame temperature at thestoichiometric interface between the exiting product stream and theexiting heated cooling air. For reduced-NOx operation, stoichiometricinterface flame temperatures should be reduced to a value less thanabout 3300 degrees F., preferably less than about 3100 degrees F., andmost preferably less than about 2900 degrees F., temperatures at whichNOx formation is greatly reduced. For the situation where about 50percent of the heat of reaction is conducted to the cooling stream, asmay be the case in a backside cooled catalyst system with sufficientcooling air such that an overall equivalence ratio of 0.5 would resultupon combining the product stream and cooling fluid stream, calculationsshow that with methane as fuel and greater than 90 percent oxygenconsumed in the catalytic reactor, an inlet temperature of 750 F. and afuel-rich equivalence ratio of 1.5 yields an adiabatic flame temperaturebelow 3300 degrees F. at the stoichiometric interface between theproduct stream and the cooling fluid stream. Similarly, a fuel-richequivalence ratio of 1.1 yields a stoichiometric interface flametemperature below 2900 degrees F. The calculations also assume catalystselectivity to the full oxidation products CO₂ and H₂O, withoutformation of H₂ or CO.

[0039] The apparatus of the present invention is designed to perform thepreviously described method. The apparatus uses conduits adapted forconducting fluid and positioned within a housing. The conduits' fluidconduction defines a cooling path whereas the exterior of the conduitsdefine a flow path within the housing. A catalyst is deposited withinthe flow path. The conduit exit peripheries define the exit from theflow path and the flow path exits and the conduit exits are collocatedand interspersed. While the embodiments depicted herein use elementshaving circular cross-sections, circular cross-sections are not requiredand should not be considered limiting unless specifically indicated.

[0040] In the first embodiment, a housing is subdivided by a plate intoa first and second zone, thereby creating two zones that are not influid communication. The housing defines an aperture in fluidcommunication with the second zone. Conduits are then placed within thehousing penetrating through the plate such that the conduit entrancesopen into the first zone and the conduit exits are in the second zonedownstream of the aperture. Upstream and downstream are defined by thenormal flow of a fluid through the invention. The exterior surfaces ofthe conduits define the flow path within the housing. This structurepermits the cooling fluid to enter into zone one and pass through theconduits and a fuel/air mixture to enter zone two through the apertureand traverse the flow path. The conduit exits and the flow path exitsare collocated and interspersed so that the fluid streams exiting bothwill mix.

[0041] The specific cross-sections and placement of the conduits withinthe housing define the contours of the flow path. The flow path,however, must allow for the diffusion of the fuel/air mixture enteringthrough the aperture throughout the relevant portion of the housing sothe fuel/air mixture can traverse through all relevant downstream areasof the flow path. In an embodiment employing a single aperture, this isaccomplished by placing the conduits within the housing such thatimmediately downstream of the aperture there are gaps between theconduits, a dispersion area, that permit the relatively unrestrictedflow of the entering fluid around the conduits. Downstream of thedispersion area the flow path can either be partitioned, unpartitioned,or a combination.

[0042] The exit from the flow path and the conduits are collocated andinterspersed. This structure introduces immediate small-scale mixing ofthe cooling fluid stream and the product stream, and permits the twoflows to mix naturally by such mechanisms as entrainment andinterspersion. The flow path exit, or exits, is defined by the conduitexit peripheries. In the preferred embodiment, flaring of the conduitexits is employed. Flaring provides a structural means to position theconduits within the housing, while permitting a gap to exist between theconduits within the flow path upstream of the exit, and provides aconvenient method of cooling the positioning structure. Other structurescould be employed and the invention should not be considered asrequiring flared conduits.

[0043] In the preferred embodiment, the catalyst is backside cooled.Backside cooling means that the catalyst is positioned on a surface inheat exchange with another surface. In the case of the preferredembodiment where the catalyst is deposited on a conduit made of metal, aportion of the heat of reaction is conducted from the surface on whichthe catalyst is deposited to the opposite surface, which is in contactwith the cooling fluid stream.

[0044] The requirement for backside cooling of the catalyst should notbe considered as limiting the invention in the sense that only backsidecooled catalyst is permitted. Non-backside cooled catalyst is permittedas long as a requisite material limit is not exceeded. Any catalyticmeans can be used to make the flow path catalytic, such as depositingcatalyst (active ingredient) onto a surface (substrate), constructing astructure from a material containing a catalyst, constructing astructure from a catalytic material, or using pellets. In the preferredembodiment, the conduit is considered a substrate and the catalyst,active ingredient, is deposited on the exterior surface. Suitablecatalyst are well known in the art.

[0045] A plenum could be added to the invention upstream of the apertureto provide further distribution of the fuel/air mixture prior to themixture entering the manifold. If a plenum is employed multipleapertures would be desired. Where multiple apertures are used, thedispersion area of the flow path could be more restricted.

[0046] The second embodiment is in essence the first embodiment with asimplified structure. In the second embodiment, the requirement for afirst zone and a plate are eliminated from the invention by modifyingthe housing. In this embodiment, the conduits merely penetrate thehousing so that the conduit entrances open to an area that is not withinthe housing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047]FIG. 1 is a schematic representation of the basic method of thepresent invention.

[0048]FIG. 2 is a schematic representation of the basic method of thepresent invention employed in a gas turbine.

[0049]FIG. 3 shows a longitudinal cross-section of the first alternativeembodiment of the present invention.

[0050]FIG. 4 shows a cross-sectional view of the flow path in the areaof the aperture of the present invention depicted in FIG. 3 lookingdownstream.

[0051]FIG. 5 shows a cross-sectional view of the flow path downstream ofthe aperture of the present invention depicted in FIG. 3 lookingdownstream.

[0052]FIG. 6 shows an end view of the catalytic reactor at the reactordischarge looking upstream.

[0053]FIG. 7 shows a longitudinal cross-section of the present inventiondepicting an unpartitioned flow path.

[0054]FIG. 8 shows a longitudinal cross-section of the present inventiondepicting a partitioned flow path.

[0055]FIG. 9 shows a longitudinal cross-section of the present inventiondepicted in FIG. 3 with a plenum.

[0056]FIG. 10 shows a longitudinal cross-section of a second embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0057] More particularly, there is shown in FIG. 1 a cooling fluidstream 30 comprising air entering a heat exchanger 2 whilesimultaneously a fuel-rich fuel/air mixture 37, comprised of a firstfluid 32 comprising air and a first fuel 33, is entering catalyst 3. Thefirst fuel 33 within fuel-rich fuel/air mixture 37 upon entering thecatalyst 3 is partially oxidized creating a heat of reaction and aproduct stream 31. The cooling fluid stream 30 absorbs at least aportion of the heat of reaction 39. The resulting product stream 31 andcooling fluid stream 30 are then contacted creating non-homogenousmixture 35. A critical feature of this present method is that thecooling fluid stream 30 be of sufficient flow rate to create a fuel-leanfuel/air mixture if mixed with the product stream 31.

[0058] The cooling fluid stream 30 can absorb the heat of reactionthrough multiple mechanisms. One method is to use the cooling fluid tocool the catalyst and associated substrate, for example backsidecooling. Another method would be to use a heat exchanger downstream ofthe catalyst.

[0059]FIG. 2 shows the general method described above in the specificapplication of a gas turbine. This specific application adds to thebasic invention described above a mixing step and a gas-phase combustionstep. An alternative application in a gas turbine could add to the basicinvention a gas-phase combustion step without prior mixing.

[0060] In the gas turbine application shown, the compressor 60compresses third fluid 36, which comprises air. The third fluid 36 isthen split into two separate streams, first fluid 32 and cooling fluidstream 30. Fuel 33 is then mixed in sufficient quantity into first fluid32 to create fuel-rich fuel/air mixture 37. Then as in the basic method,a portion of the fuel 33 within the fuel-rich fuel/air mixture 37 isthen oxidized by catalyst 3 creating a heat of reaction and productstream 31. A portion of the heat of reaction 39 is extracted intocooling fluid stream 30 as it passes through the heat exchanger 2. Theproduct stream 31 is then contacted with cooling fluid stream 30 tocreate non-homogenous mixture 35. Non-homogenous mixture 35 is thenmixed to create fuel-lean fuel/air mixture 38. Fuel-lean fuel/airmixture 38 is then conducted into a combustion zone 62 where gas-phasecombustion occurs. The resulting combustion products 74 are thenconducted into turbine 61. In the gas turbine application, the thirdfluid 36 can be additionally used as a source for dilution air (notshown) upstream of turbine 61.

[0061] Mixing of non-homogenous mixture 35 to create fuel-lean fuel/airmixture 38 without premature inflammation requires that known flameholding methods be avoided. Use of the apparatus disclosed herein isadvantageous.

[0062]FIG. 3 shows a longitudinal cross-section of the first alternativeembodiment of the present invention. In this embodiment, the apparatuscomprises a catalytic reactor 100 comprised of a housing 102 having anentrance and an exit, and defining at least one aperture 107. A plate115 is positioned within the housing 102 defining a first zone 105 and asecond zone 106. The aperture 107 is in fluid communication with thesecond zone 106.

[0063] At least two conduits 110 made from a heat conducting materialand adapted for conducting a fluid are positioned within the housing102. The conduits have an entrance 116, an exit 117 with an exitperiphery 113, an interior surface 112, and an exterior surface 111. Theconduits 110 are positioned within the housing 102 such that theconduits 110 penetrate plate 115 thereby having the conduit entrances116 in fluid communication with the first zone 105 and the conduit exitswithin the second zone 106. A first fluid 120 entering first zone 105must enter second zone 106, if at all, by exiting conduits 110. Theconduit exit periphery 113 positions the conduits 110 relative to eachother and the housing interior surface 114.

[0064] The flow path 123 within housing 102 is defined by the conduitexterior surfaces 111. The flow path extends between the aperture 107and the flow path exits, which are defined by the conduit exitperipheries 113. The flow path 123 can have numerous physicalconfigurations that are application dependent. In general, the flow pathmust permit the diffusion of the entering second fluid 127 in a mannerto ensure the second fluid 127 can enter all the passages containingcatalyst downstream therefrom. Those skilled in the art will appreciatethe numerous structures that can be designed based upon the specificapplication, thus the invention should not be considered limited to theflow paths depicted in the embodiments presented.

[0065]FIG. 3 depicts a partitioned flow path. Just downstream of theaperture 107, the flow path 123 allows for the second fluid 127 todisperse throughout housing 102. Further downstream however, the flowpath has been subdivided into a plurality of smaller passages.Partitioned means that the fluid is essentially confined to the smallerpassages. Partitioning is accomplished by physical means, such as asolid barrier or by contact (close proximity) of surfaces. In thisembodiment, the subdivision into a plurality of small passages isaccomplished by contact, expanding the cross-section of the conduits 110so that they touch.

[0066] A catalyst 103 has been deposited on a portion of the conduitexterior surface 111. Catalyst can be deposited anywhere in the flowpath. It is preferred that the catalyst be deposited downstream ofaperture 107.

[0067]FIG. 4 is a cross-sectional view of the housing 102 taken throughaperture 107 looking downstream showing the definition of the flow path123 by the conduit exterior surfaces 111 within housing 102. To allow asecond fluid 127 upon entering the second zone to diffuse, thecross-sections of the conduits 110 are sized to permit the fluid toeasily flow around the conduit exterior surfaces 111.

[0068] As shown in FIG. 5, which is a cross-sectional view of thehousing 102 approximately mid-way between the aperture 107 and the flowpath exits 125, the conduit 110 cross-sections have been sized such thatthe conduit exterior surfaces covered with catalyst 103 touch, or nearlytouch, one another or the housing interior surface 114. The sizing ofthe conduit 110 cross-sections in this manner effectively divides theflow path 123 into a plurality of passages.

[0069]FIG. 6 shows an end view of the catalytic reactor 100 lookingupstream from the discharge end of the catalytic reactor 100. Theconduit exit peripheries 113 define the flow path exits 125 as well asassure the conduit exits 117 are interspersed with the flow path exits125. In this embodiment, the conduit exit peripheries 113 provide thestructure which holds the conduits 110 in position by contacting thehousing interior surface 114 within the housing 102.

[0070]FIG. 7 shows a longitudinal cross-section of another embodiment ofthe present invention. This embodiment is the same as that depicted inFIG. 3 except that the flow path 123 is of a different configuration. Inthis embodiment, the flow path is unpartitioned. Unlike the embodimentdepicted in FIG. 3, the conduit cross-sections are sized to allow thesecond fluid 127 to flow around the conduits throughout the entirelength of the flow channel 123. The flow path after the initialdispersion area can be partitioned, unpartitioned, or a combination. Inthe embodiment shown in FIG. 7, the conduit exit peripheries 113 definethe flow path exits 125 as well as assure the conduit exits 117 areinterspersed with the flow path exits 125. In this embodiment, theconduit exit peripheries 113 provide the structure which holds theconduits 110 in position by contacting the housing interior surface 114within the housing 102. While flares are shown, it is not required andthe invention should not be considered so limited.

[0071]FIG. 8 shows a longitudinal cross-section of another embodiment ofthe present invention. This embodiment is the same as that depicted inFIG. 3 except that the flow path 123 is partitioned by a physicalbarrier. The conduit exterior surfaces are integrated into a structureresembling a monolith. In this embodiment the flow path 123 is stillconsidered defined by the conduit exterior surfaces 411, and thecatalyst 103 is considered deposited thereon.

[0072]FIG. 9 shows a longitudinal cross-section of the embodimentdepicted in FIG. 3 with a plenum 130 added upstream of the aperture 107and in fluid communication 10 therewith. If a plenum 130 is employedmultiple apertures 107 are preferred. A plenum 130 can be incorporatedinto any of the previously discussed embodiments.

[0073]FIG. 10 shows a longitudinal cross-section of another embodimentof the present invention very similar to that disclosed in FIG. 3. Thisembodiment, however, is based on a simplified housing structure. In thisembodiment, the catalytic reactor 200 comprises a housing 202 having anexit, and defining at least one aperture 207.

[0074] At least two conduits 210 made from a heat conducting materialand adapted for conducting a fluid are positioned within the housing202. The conduits have an entrance 216, an exit 217 with an exitperiphery 213, an interior surface 212, and an exterior surface 211. Theconduits 210 are positioned within the housing 202 such that theconduits 210 penetrate the housing thereby having the conduit exitswithin the housing 202 and the conduit entrances 216 opening to an areaoutside the housing 202. A first fluid 220 entering conduits 210 entershousing 202, if at all, by exiting conduits 210. The conduit exitperiphery 213 positions the conduits 210 relative to each other and thehousing interior surface 214.

[0075]FIG. 10 depicts an unpartitioned flow path. This embodiment,however, has all the flexibility of the first embodiment. As with thefirst embodiment a plenum could also be incorporated.

[0076] For application in a gas turbine, the catalytic reactor must beintegrated into the gas turbine combustion system. For gas turbineengines using a combustor shell to contain the high-pressure gaseswithin the combustion section and to provide a sealed flow path fromcompressor exit to turbine inlet, the reactor housing is relieved of theneed to contain high pressure. The fuel-rich fuel/air mixtureadvantageously should be uniformly mixed prior to delivery to the flowpath. Mixing of fuel and air within the flow path is also feasible ifthe reactor is designed accordingly.

[0077] As a general design rule, it is desirable to design the catalyticreactor such that the catalytic reaction approaches its maximum possibleextent at all expected operating conditions, so that variations inchemical reaction rates and mass transfer rates do not affect thecatalytic reactor output. Thus, sufficient catalyst coating should beapplied that O₂, the limiting reactant, is substantially consumed in theflow path. O₂ conversions greater than 50 percent are preferred, and O₂conversions greater than 75 percent are most preferred.

[0078] Sufficient catalyst coating means sufficient loading, on a weightbasis, as well as sufficient geometric surface area of catalyst.Insufficient loading will result in an insufficient number of catalyticreaction sites, and insufficient geometric surface area will result ininsufficient total mass transfer from the gas-phase to the catalyticsurface. In either case, insufficient catalyst means that O₂ conversionswill be below the preferred levels. The required loading and therequired geometric surface area will depend upon operating conditions(e.g. reactant temperature, pressure, velocity, composition) andcatalyst activity, and can be determined by methods known in chemicalengineering practice.

[0079] The catalyst coating used in the present invention, where thefuel is a hydrocarbon and oxygen is the oxidizer, may have as an activeingredient precious metals, group VIII noble metals, base metals, metaloxides, or any combination thereof. Elements such as zirconium,vanadium, chromium, manganese, copper, platinum, palladium, osmium,iridium, rhodium, cerium, lanthanum, other elements of the lanthanideseries, cobalt, nickel, iron, and the like may be used. The catalyst maybe applied directly to the substrate, or may be applied to anintermediate bond coat or washcoat composed of alumina, silica,zirconia, titania, magnesia, other refractory metal oxides, or anycombination thereof.

[0080] The catalyst-coated substrate may be fabricated from any ofvarious high temperature materials. High temperature metal alloys arepreferred, particularly alloys composed of iron, nickel, and/or cobalt,in combination with aluminum, chromium, and/or other alloying materials.High temperature nickel alloys are especially preferred. Other materialswhich may be used include ceramics, metal oxides, intermetallicmaterials, carbides, and nitrides. Metallic substrates are mostpreferred due to their excellent thermal conductivity, allowingeffective backside cooling of the catalyst.

[0081] Fuel types include hydrocarbons, hydrocarbon oxygenates, andblends thereof. Suitable gaseous fuels include natural gas, methane, andpropane. Suitable liquid fuels include gasoline, kerosene, No. 1 heatingoil, No. 2 heating oil, and conventional aviation turbine fuels such asJet A, Jet B, JP-4, JP-5, JP-7, and JP-8. “Hydrocarbon” not only refersto organic compounds, including conventional liquid and gaseous fuels,but also to gas streams containing fuel values in the form of compoundssuch as carbon monoxide, organic compounds, or partial oxidationproducts of carbon containing compounds. If the fuel is a liquid, itshould be vaporized or atomized before mixing with air or while beingmixed with air.

EXAMPLE 1

[0082] A catalytic reactor similar to that illustrated in FIG. 9 wasfabricated for dual air-source testing, with separate air flow controlsfor the flow path and the conduits. A single fuel source was employed.As shown in FIG. 9, a plenum supplied the fuel-rich fuel/air mixture tothe flow path through multiple apertures. At the downstream end of thecatalytic reactor, the product stream exited the flow path via theinterstitial space created by the conduit peripheries. The cooling airexited the conduits at this same axial location, and mixed with theproduct stream.

[0083] The conduits, specifically tubes, were 10 inches in length withan outside diameter of 0.188 inches, and a material thickness of 0.010inches. One end of the tube was expanded at a constant angle of 4degrees until the cross-section was increased about 30 percent, to afinal inside diameter of 0.255 inches. A flat segment was provided onthe 0.255-inch-diameter flared section of about 0.1 inches length. Thehousing was sized such that seven tubes could be accommodated andpositioned therein by the flares. The tubes were inserted through theplate and brazed thereto to form a tight seal.

[0084] A catalyst was deposited on approximately 8.5 inches of theexterior of the tubes. To prepare for catalyst application, an aluminawashcoat was first applied, with a loading of approximately 20 to 40mg/square-inch. Palladium catalyst was then applied to the washcoat,with a loading of approximately 10 to 15 mg/square-inch. There was somevariation in both washcoat and catalyst loading.

[0085] The catalytic reactor was installed in a refractory-linedcylindrical pressure vessel to permit testing of the catalytic reactorat pressure. A fuel/air inlet pipe penetrated the vessel wall through ahigh-pressure fitting, and mated with a sealing fitting at the fuel/airinlet plenum of the catalytic reactor. Cooling air was supplied to theconduits by a separate line which entered the pressure vessel at itsupstream end. Upon exiting the catalytic reactor, the combustible gasmixture (the combined product stream and the cooling stream) entered a0.495-inch inside-diameter extension tube, followed by a nozzle blockthat tapered down to a 0.375-inch inside-diameter at its exit. The totallength from the conduit exits to the downstream end of the nozzle blockwas approximately 15 inches. Immediately downstream of the nozzle blockexit was a sudden expansion to a 3 inch-diameter burnout zone forcombustion completion.

[0086] At 10 atm pressure, the catalytic reactor was operated at aninlet reference velocity of 250 ft/s. The inlet reference velocity isdefined as the velocity which would result inside the catalytic reactorhousing without the conduits. In other words, if all fuel and airentering the catalytic reactor (including both the conduit cooling airand the fuel-rich fuel/air mixture) were mixed before reaction to forman aggregate mixture at an aggregate temperature and mass flow rate, andif this aggregate mixture had a uniform velocity throughout the reactor,and if the conduits were of zero thickness, then the velocity inside thereactor housing would be 250 ft/s.

[0087] At the 10 atm, 250 ft/s inlet reference velocity condition, 10percent of the total air was delivered to the flow path, and 90 percentof the total air was delivered to the conduits for cooling. The fuelflow rate was set to provide an overall 0.5 equivalence ratio in thefuel-lean fuel/air mixture downstream of the catalyst, giving anequivalence ratio of 5.0 for the fuel-rich fuel/air mixture. The coolingair was heated to 950 degrees F. at the catalytic reactor inlet. Thefuel-rich fuel/air mixture entered at room temperature (nominally 60degrees F. ). The resulting overall adiabatic flame temperature in thedownstream burn-out zone was approximately 2800 degrees F. NOx emissionsof less than 5 ppmv (corrected to 15 percent excess O₂ dry) weremeasured from the downstream sampling port (14 inches downstream of thesudden expansion plane), indicating that all burning took place in awell-mixed mode at flame temperatures in the vicinity of 2800 degrees F.As desired, there was no high-NOx-producing combustion during mixing ofthe cooling stream and the product stream. In this configuration atthese conditions, the conduit exits act as multiple jets surrounded by aco-flowing product stream. The jets, nominally 0.255 inches in diameter,allowed rapid mixing at this small scale and helped to prevent ignitionand burning of the reactants within the product stream before mixing wasachieved. At the conditions given, the maximum catalyst substratetemperature was measured to be below approximately 1800 degrees F.,which is below the substrate and catalyst material failure point. Gassampling from the downstream end of the flow path indicated thatapproximately 90 percent of the O₂ present in the fuel-rich fuel/airmixture was consumed prior to exiting the flow path.

[0088] These results confirm that the method and apparatus of thepresent invention are capable, at gas-turbine-type operating conditions,of providing the desired result: fuel-rich catalytic reaction followedby stable, low-NOx gas-phase combustion, with well-moderated catalystoperating temperatures.

[0089] Although the invention has been described in considerable detail,it will be apparent that the invention is capable of numerousmodifications and variations, apparent to those skilled in the art,without departing from the spirit and scope of the invention.

What is claimed is:
 1. An assembly comprising: a) a header plate; b) twoor more conduits of a heat conducting material and adapted forconducting a fluid, the conduits having an entrance, and exteriorsurface, and an exit with a periphery, such that: the conduits penetratethe plate and are connected thereto whereby the conduit entrances areseparated by the plate from the conduit exits; c) catalytic means formaking at least a portion of the conduit surfaces between the plates andthe exits catalytically active. surface, and the catalytic means ispositioned on at least portion of at least one of the exterior surfacessuch that there is a heat exchange relationship with the interiorsurface.